A Novel Electrostatic Radio Frequency Micro Electromechanical Systems (RF MEMS) with Prognostics Function

A Novel Electrostatic Radio Frequency Micro Electromechanical Systems (RF MEMS) With Prognostics Function

Yunhan Huang, Michael Osterman, and Michael Pecht Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, MD 20742, United States

most widely researched RF MEMS is electrostatic capacitive Abstract RF MEMS switch. Capacitive RF MEMS can provide lower Radio Frequency Micro Electromechanical Systems (RF return loss, higher isolation, zero power consumption, and MEMS) has emerged as one of the most promising front higher linearity than integrated circuit (IC) counterpart runners in wireless components market because of their high technologies, e.g., SOS. They can be used as relays, linearity, high isolation, ultra-low power consumption, and resonators, tunable capacitors, reconfigurable antenna, phase the capability of integrating with integrated circuits for shifter, and tunable filters [1-4]. Additionally, their capability portable wireless communication devices. However, their of integration with IC to form one monolithic RF front-end widespread application in commercial areas is hampered by chip for multiband wireless communication devices can yield the relatively poor reliability performance during long-term a higher space efficiency for multiple bands e.g., GSM, 3G, usage. Among the failure mechanisms of RF MEMS, stiction LTE, Bluetooth, Wi-Fi, FM, etc.. Some drawbacks, such as induced by the charge accumulation in the dielectric layer is costly packaging and integration with IC and required high the predominant one, accounting for most of the failed operating voltage that were hampering the commercialization components. However, the origin of the accumulated charge, of RF MEMS has been greatly overcome by the development its properties and distribution, and its adverse effect on of MEMS packaging and use of integrated charge pump. devices’ electrical performance has not yet been fully However, RF MEMS’s relatively poor reliability performance understood. is still the most challenges of the commercialization of RF In this paper, we propose the design, realization, MEMS. The lifetime of a typical RF MEMS can hardly characterization, and reliability test of a novel RF MEMS compare with that of IC counterparts. In order to improve the capacitive switch which has a high RF performance and low device’s lifetime, researchers have conducted reliability test fabrication cost with a capability of predicting its state of and failure analysis and identified the predominant failure is health for various applications from phase shifters to tunable stiction, is a phenomenon that adjacent micro structures get antennas. The key characteristic of our design is the stuck when they come into contact and restoring forces are introduction of Prognostics and Health Management (PHM) not great enough to overcome the surface adhesion forces. using non-intrusive monitoring method, which allows us to calculate the remaining useful life of our RF MEMS 2. RF MEMS Design, Fabrication and Characterization capacitive switches and provide a warning before its onset of As early as 1979 [5], micro-electromechanical switches failure. We overstress the device using two methods: have been developed and used as a switch to turn on and off electrostatic discharge (ESD) and operational voltage low-frequency electrical signals. According to the types of waveform. We discovered the difference of RF MEMS contact between the suspended movable part and the behavior and lifetime. We also present the effect of driving underneath electrodes, RF MEMS switches can be grouped voltage polarity on the lifetime of RF MEMS. into two types: the capacitive switch and the ohmic switch. In the capacitive switch the two electrodes are electrically separated by a dielectric layer. In this work, we focus on 1. Introduction capacitive RF MEMS. The switch designs have utilized The development of the wireless communication has led to cantilevers [6], rotaries [7] and membranes [8] to achieve the massive growth of radio frequency (RF) and microwave good performance at RF and microwave frequency range. circuits and systems for various applications, including Because of their intrinsic low loss, low power consumption wireless hand-held devices, wireless local area networks, and lack of intermodulation distortion, RF MEMS switches satellite communications and radar. Future hand-held are an attractive alternative to traditional FET or p-i-n diode communication devices will require very low weight, volume switches in applications where microsecond switching speed and power consumption in addition to higher data rates and is sufficient. Membrane-based switches operated by improved RF functionality, e.g., higher isolation, lower return electrostatic force generated by driving voltage have shown loss. Improvements in the size and component count have excellent performance through 40GHz [8], fast switching been achieved by increasing the level of integration. There is speed, and lifetimes in excess of 1 billion cycles, which is one an urgent need to reduce the size and power consumption of of the most promising configuration of RF MEMS [9]. A RF frontend circuit to extend the battery lifetime, especially schematic of a typical capacitive RF MEMS is shown in Fig. for mobile communication devices or wireless sensor 1. network, while maintaining RF performance. RF-MEMS stands for radio-frequency micro electromechanical systems. They comprise an excellent choice for future wireless communication devices. One of the

Fig. 3 The RF MEMS switch under microscope [10].

Fig. 1 Schematic of a typical capacitive RF MEMS switch consisting of a metal membrane (the bridge structure), a transmission line underneath (the central line) covered by insulation material (blue area).

2-1 RF MEMS Design The basic operation of the capacitive RF MEMS switch discussed here is a co-planar waveguide (CPW) shunt switch as shown in Fig. 1. The RF MEMS switch operates as a tunable capacitor with bi-states. The input signal waveform is a combination of DC bias (square wave) and RF signal Fig. 4 The equvilent electric circuit of a capacitive RF MEMS (sinusoidal wave). The DC bias frequency should be lower switch. The tunable capacitve C is formed by the movable than the mechanical resonance frequency of the membrane, membrane and the transmission line underneath. (for most RF MEMS, 10~100 kHz); DC bias voltage should be higher than the pull-down voltage, which is the minimal voltage to pull the membrane down, contacting the underlying dielectric layer. The RF signal frequency is much lower than the resonance frequency of the electrical circuit. With a DC bias that is higher than the pull-down voltage applied, the membrane is pulled down and touches dielectric, a high capacitance is induced due to the reduction of the separation between the membrane and the underlying transmission line.

Thus, a low impedance is achieved at RF frequency range and Fig. 5 A linear electromechnaical model of capacitive RF the RF signal is shunted to ground in shunt switch and MEMS. The membrane is subjected to an electrostatic force directed to output in series switch. If only a RF signal is when a bias voltage is applied and a mechanical restoring applied without any DC bias, the membrane stays up and does force due to the displacement of the membrane. not respond (vibrate) to the RF signal due to that the mechanical resonance frequency of the membrane is lower The schematic of the 1-D linear electromechanical model than the RF signal. Thus, a low capacitance and high of our RF MEMS with and without a DC bias is shown in Fig. impedance is induced, directing the RF signal to the output in 2. If we assume the membrane is subjected to a distributed a shunt switch and to ground in a series switch. The force across the entire membrane, then the spring contant can equivalent circuit of a typical RF MEMS can be modeled as a be found RLC circuit consisting of a serial resistance R , inductance L, s t 3 and a tunable capacitance C (shown in Fig. 4). The k  32EW ( ) (2) L impedence can be written as Z=Rs + jωL + 1/jωC, and where E is the Young’s Modulus of the material, W, t, and L Z=1/jωC for f<

signal line is modeled as a parallel-plate capacitor. Given that the width of the signal line is W, the parallel plate capacitance is  LW C  0 (3) g  z  t /  r where g is the initial gap between the membrane and the dielectric layer, ∆z is the displacement of the membrane. The

electrostatic force induced as charge, provides a capacitance, Fig. 2 Schematic diagram showing the operation of a RF given by [12] MEMS switch [10].

122 V 2 dC(g) WwV 2 Al SiO2 F    (4) e 2 dg 2g 2 where V is the voltage applied between the membrane and the signal line. Where the force is independent of the voltage polarity. The electrostatic force is evenly distributed across the section of the membrane above the signal line. Therefore, Al the appropriate spring constant can be used to determine the distance that the membrane moves under the applied force. The pull-down voltage can be found from [13] 8kg 3 V  eq (5) p 27 WL where Vp is pull down voltage, k is the spring constant of membrane, g is the equivalent gap between membrane and signal line. When the RF MEMS is in its initial undisplaced position (switch-off), the gap consists of two dielectric materials namely air and SiO2. The dimension of the MEMS switch is shown in Table 1. Fig. 6 The diagram showing our fabrication process. (1)

Deposition and patterning of coplanar waveguide. (2) Component Material Dimension Depositon and patterning of silicon dioxide. (3) Spin coating MEMS bridge Aluminum t= 0.5 μm and patterning of sacrificial layer—photoresistor polymer. (4) L=150 μm Deposition and patterning of a aluminum bridge. (5) Removal W=200 μm of the sacrificial layer and relasing the membrane [10]. Signal line Aluminum Thickness=1 μm A chemical composition analysis was performed to check if W=100 μm there was any carbon element (photoresistor) left out in Dielectric layer SiO2 gSiO2 =1.4 μm between the membrane and the dielectric layer. If there is Air gap Air photoresistor, it may serve as an adhesive to cause the gair =1 μm membrane stick to the dielectric, which is refered to as GSG gap Air G=25 μm stiction. Table 1 shows the dimension of the MEMS switch. 2-2 RF MEMS Fabrication The fabrication process of the RF MEMS switch is shown in Fig. 6. Arrays of RF MEMS structures were fabricated by first depositing a thin aluminum electrode to serve as the signal line for the switch. A silicon dioxide isolation dielectric layer is deposited on top of the electrode to to enable the RF MEMS switch capacitor when the membrane is pulled down. The membrane is deposited over a sacrificial polymer layer that is released at the end of the surface micromachining process. After sacrificial layer is released, the aluminum Fig. 7 Energy dispersive spectroscopy (EDS) analysis of the membrane is left suspended over the dielectric layer. Its top surface of dielectric and the bottom surface of membrane natural state is in the “up” or unactuated position. When a after the membrane is peeled off. sufficient DC electrical potential is applied between the membrane and electrode, the membrane snaps down into the 2-3 RF MEMS Characterization actuated state. RF MEMS switches were fabricated using The electromechanical and RF signal properties of the dimensions listed in Table 1, and shown in Fig. 2. The metal MEMS is characterized after fabrication. A ground-signal- membrane was fabricated using aluminum because of its high ground (GSG) probe having 250 micro pitch with resistance to fatigue and low electrical resistance. A 1 micron, appropriate Bias Tee and a Network Analyzer are used to nano-porous SiO dielectric isolation layer which separate the 2 make time domain reflectometry (TDR) analysis of the RF membrane and signal line was deposited using a Uni-axis MEMS to extract S11 insertion loss parameters and paracitics PECD machine . After fabricating the RF MEMS structure, a (Fig. 8). The Bias Tee is used to apply DC bias power to the layer of photoresist was deposited on the surface, to protect signal line. the wafer from debris and damage during dicing. We found that the pull-down voltage required to change the capacitance of the switch was around 30 volts. The

123 resonance frequency of our RF MEMS is measured 23.75 We tested RF MEMS under different ESD (Human body kHz. model) voltage level and found that pulses higher than 2kV can cause burnout of both membrane and dielectric layer.

Fig. 8 The GSG probe was connected to a RF MEMS switch [10].

a) Fig. 11 Scanning electron microscope (SEM) image shows the burnout of RF MEMS after ESD tests.

For a lower ESD voltage, the primary failure is charge- induced stiction.

Fig. 9 (a) Mechanical resonance frequency of RF MEMS measured by laser vibrometer. (b) the modal analysis at 1st resonance frequency.

3. Reliability of RF MEMS Switches and PHM Implementation In order to estimate the state of health of RF MEMS, we conducted two types of overstress tests: (1) ESD test and (2) operational voltage test. We characterized the RF MEMS switches before any test, then we subject those switches to either ESD (Human body model) pulses or 60V unipolar Fig. 12 The shift of C-V curve indicates the charge square waveform. We measured capacitance-voltage (C-V) accumulation in dielectric after several ESD pulses. curve intermittently and looked at the membrane with a non- contact surface profiler. We used surface profiler to prove that the stiction occurred.

a) b) a)

c) d)

Fig. 10 The overstress of RF MEMS using ESD and 60V square waveform. (a) is the ESD simulator that generates a ESD pulse shown in (c). (b) is the controllable power supply generates square waveform shown in (d).

124 a) b)

b) Fig. 13 surface profiler data shows after 20 ESD pulses, the membrane was stuck to the dielectric.

For operational voltage tests using 60V, 0.5 Hz, square waveform, the primary failure is stiction shown by surface profiler. No burnout was observed.

initial 0.6 after 2min (60 cycles) after 10min (300 cycles) after 60min (1800 cycles)

0.5

0.4 Fig. 14 The shift of C-V curve of RF MEMS overstressed under ESD test (a) and operational voltage test (b). ESD causes C-V shift even after one pulse, whereas in operational 0.3 voltage tests the shift of C-V curve is exponential with the Capacitance (pF) number of cycles. 0.2 By analyzing the shift the C-V curve, the accumulated

0.1 charge density in the dielectric layer can be found. 4. Conclusions 0 -30 -20 -10 0 10 20 30 In this paper we proposed the design, realization, Bias Voltage (V) characterization, and reliability test of a novel RF MEMS Fig. 13 The shift of C-V curve indicates the charge capacitive switch. We developed a non-intrusive monitoring accumulation in dielectric after operating cycles. The black method using surface profiler to assess the amount of charge curve shows stiction occurs. trapped in RF MEMS switches. The key characteristic of our By using the data of surface profiler, we can measure the design is the introduction of Prognostics and Health displacement of membrane of RF MEMS. Since the Management (PHM) that allows us to calculate the remaining displacement is caused by electrostatic force due to dielectric useful life of our RF MEMS capacitive switches and provide charge, the amount of charge can be estimated, and in turn, a warning before its onset of failure. We verified the method the state of health and remaining useful life of RF MEMS can by overstressing our devices under both electrostatic be predicted. The algorithm to predict the onset of failure will discharge (ESD) and operational voltage waveform. be presented at ECTC. Acknowledgments The authors thank the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland for supporting this reserach. CALCE provides equipment and resources to support the study of energy harvesting technology. More than 100 national and international organizations and companies support the Center. Also, the authors would like to acknowledge Dr. Ravi Doraiswami, Prof. Reza Ghodssi, Prof. Balakumar Balachandra for their valuable help.

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